Without Azure App Registration you need to use the implicit type of grant.
The documentation provides a step-by-step tutorial on how to do:
https://learn.microsoft.com/en-us/power-apps/developer/data-platform/webapi/use-postman-web-api
I have no idea if it's possible but I was wondering why would you need such thing, can you please elaborate on that ?
If you have a username and password it means that you might have a license for the product which means that you can access CRM. Why would you need an API when you can access CRM and when you are an INTERNAL user ?
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